Have you considered moving to Motorola PocketBrowser? PocketBrowser wraps the IE commponent on your MC3090 and enables you to specify an error page for navigation problems.
PocketBrowser (PB) was our first choice, but proved very expensive, despite our very high purchase volumes, although I have evaluated PB.
PB is based on PIE, IE6 for CE seems somewhat faster, the challenge of course is 2-way communication between a managed-code shell app and client markup in the control.
I have achieved this using #tags appended to the URL - scanning in managed code with the browser control displaying markup - works a treat.
If its of interest to anybody, I use VS2010 MVC3 server-side.
Client-side error handling is my only remaining obstacle.
Again, any help with original question much appreciated.